1. Field of the Invention
This invention relates generally to image display systems and more particularly to a method and apparatus for storing and fetching image data to be presented on a display screen through interlaced scanning.
2. Description of the Related Art
Televisions are the centerpiece of household entertainment. The functionality provided by televisions is constantly being expanded. More recently, with the blurring of the boundaries between computers and televisions, televisions are acquiring more qualities of computers and vice-versa. Additionally, televisions are moving from analog processing to digital processing. With the convergence between the devices, greater demands are placed on the systems. Thus, more data is being processed by the systems and the demands on the system memory are increasing, thereby requiring a higher bandwidth to avoid image corruption. Additionally, the quality and the resolution of the displays on the television systems are constantly improving, thereby placing further processing demands on the system.
Images presented on the display screen of a television employ interlaced scanning to ensure that the picture has an even brightness throughout instead of having separate bright and dark bands. That is, each still picture is made up of two scans consisting of alternate lines. For example, the even numbered lines are scanned and then the odd numbered lines are scanned, with a complete pass through all the even and odd numbered lines defining one frame of data.
Sharp transitions in color between adjacent scan lines result in flicker when presented on a television screen. That is, flicker is caused on a National Television System Committee (NTSC) monitor screen when a pixel or horizontal group of pixels of a higher intensity are bordered on both the top and bottom by lower intensity pixels. The flicker causes a strain on human eyes as well as deterring from the quality of the display. Since bandwidth is a concern with television, interlaced scanning has been adopted to allow the frame repetition rate to be reduced by one half, as interlaced scanning scans half the lines in each vertical scan. Thus, each frame takes two vertical scans with even and odd lines scanned on alternate fields.
FIG. 1 is a simplified schematic diagram of a system for storing image data in a television's memory. Image data is stored in memory 100 as alternating even and odd lines due to the ease of use for the software that interacts with the data. A separate memory access is necessary for odd line data and even line data. That is, one memory access is performed to fetch even line data to be placed in even buffer 102, and a separate memory access is made to fetch odd line data to be placed in odd buffer 104. The even and odd line data are then averaged through flicker filter 106 and eventually output as a single line to display 104. Thus, in order to output a single pixel of display data, two memory accesses must be made to acquire the even and odd line data to be averaged from each of the two buffers.
The multiple fetches from the buffers place high demands on the memory when other devices are competing for memory. In turn, a high bandwidth is required from memory in order to keep up with the requests for data. If the memory can't support the requests for data, then image corruption on the display results in an incomprehensible display, i.e., corrupted image. Furthermore, as television systems further enhance the functionality available to the viewer and as television manufacturers simultaneously embrace high definition technology, the demands on memory will further rise. In turn, costs are incurred in supporting these demands in terms of the increased memory capacity to avoid image corruption and increased power consumption.
As a result, there is a need to solve the problems of the prior art to provide an apparatus and method for reducing the demands on the memory for a system used to generate a display through interlaced scanning, while maintaining the image quality.